1. Field of the Invention
This invention relates to a surface emitting semiconductor laser with a current confinement structure for confining a current into a restricted active region, which can be suitably used in optical information communications, optical information processing apparatuses, and recording apparatuses such as laser beam printers. This invention also relates to its fabrication method.
2. Related Background Art
A surface emitting semiconductor laser has been marked with keen interest as a light source in optical information communications and processing, and its development has been energetically advanced. The surface emitting semiconductor laser can be characterized by low electric power consumption, low threshold current, two-dimensional dense integration capability, and dynamic single mode operation.
A conventional surface emitting semiconductor laser is illustrated in FIG. 1. The laser is fabricated as follows. A p-type GaAs/AlGaAs multi-layer mirror 11 is etched into a mesa shape, and sides of the mesa mirror 11 are filled with insulating material such as polyimide 4. An anode electrode 21 is then formed as illustrated in FIG. 1. In FIG. 1, there are further formed an n-type GaAs/AlGaAs multi-layer mirror 12, an InGaAs/GaAs active region 13, a layer 14 which is selectively oxidized except for its central portion which is not oxidized, and a cathode electrode 22 formed on the bottom face of an n-type GaAs substrate 1.
In the device of the above configuration, however, its thermal resistance increases since the sides of the mesa are surrounded by the insulating material 4 having high thermal resistance. Further, the area of the electrode 21 is very small due to a very small device size, and its contact resistance is large. Thus, thermal characteristics of this device are not preferable, and its oscillation wavelength is therefore likely to shift when the device is driven. In addition, its serial electrical resistance is large, and its consumption electric power hence increases.
The following surface emitting semiconductor laser is proposed in Japanese Patent Application Laid-Open No. 7(1995)-38196 to solve the above problem. In this laser, sides of an active layer and other layers exposed by etching are covered with a material having high electrical resistance and high thermal conductivity, and those sides are surrounded by a metal to reduce its thermal resistance. Thus, an additional process is needed in this fabrication process to form a region with high electric resistance and large thermal conductivity on the side wall.
As a technique for improving the performance of a surface emitting semiconductor laser, there also exists the technique in which an AlAs layer inserted in a semiconductor multi-layer mirror is partially oxidized to form the current confinement structure. In this technique, sides of the semiconductor multi-layer mirror are exposed by dry etching, and the exposed sides are then heat-treated in water vapor. The AlAs layer is thus oxidized to a desired width in an in-plane direction (a direction perpendicular to a layering direction) with its central portion being left as is, and an insulation layer is partially formed in the AlAs layer to fabricate the current confinement structure.
Further, there exists the technique for utilizing an AlAs layer as an etch stop layer, which is disclosed in Japanese Patent Application Laid-Open No. 10(1998)-294528. In this technique for fabricating a surface emitting semiconductor laser, dry etching is conducted using an etching gas whose etching rate is low for AlAs. When the AlAs layer is used as an etch stop layer, over-etching of the AlAs layer can be prevented, and exposure of an active layer can be hence prevented. The exposure of the active layer is likely to lead to a decrease in radiation efficiency due to damage to the active layer. Accordingly, this technique is very effective and valuable.
In this technique wherein the AlAs layer is used as an etch stop layer, a mixed gas with a high etching rate for a GaAs layer and a low etching rate for an AlAs layer is used, but a ratio of etching rates between GaAs and AlAs can be sufficiently controlled by changing conditions, such as a vacuum degree, even with a conventional etching which uses a fluorine gas. Further, there is no disclosure that an oxidized AlAs layer is used as an insulating layer between p-type and n-type layers, in Japanese Patent Application Laid-Open No. 10(1998)-294528.
It is an object of the present invention to provide a surface emitting semiconductor laser whose thermal characteristic can be readily improved, whose electrical resistance can be readily reduced, whose electric power consumption for driving can be readily lowered, whose threshold current can be readily reduced, which can be fabricated by a relatively simple process, which can be readily arrayed, and which can be readily provided with a current confinement structure; and its fabrication method. This object is achieved by effectively utilizing a selective oxidization layer. (As used herein, selective oxidization layer refers to either a selective oxidizable layer that is not yet oxidized or a selective oxidized layer that is already oxidized, or both, according to contex.)
It is another object of the present invention to provide a surface emitting semiconductor laser with a built-in current confinement structure, whose fabrication yield can be readily improved by effectively utilizing a selective oxidization layer (e.g., AlxGa1xe2x88x92xAs layer (typically 0.8xe2x89xa6xxe2x89xa61)), that is already oxidized, as an etch stop layer; and its fabrication method.
The present invention is generally directed to a surface emitting semiconductor laser which includes an active region formed on a growth substrate; upper and lower mirror layers that sandwich the active region to construct a vertical cavity; a selective oxidization layer that is selectively oxidized and insulated and that is provided on the side of the active region opposite to the side of the substrate; and a current injecting unit for injecting a current into the active region. In this structure, a post portion is formed by removing semiconductor material formed on the substrate down to an uppermost or halfway level of the selective oxidization layer while the selective oxidization layer is used as an etch stop layer, and the selective oxidization layer is formed to act as both a current confinement layer for current injection and an insulating layer for the current injecting unit.
In this structure, p-type semiconductor is insulated from n-type semiconductor by the selective oxidization layer, except for a portion of a current, or devices are insulated from each other by the selective oxidization layer when a plurality of surface emitting semiconductor lasers are arrayed. Accordingly, there is no need to separately form an insulating layer, and the fabrication process can be hence simplified. Further, the current confinement structure can be formed by the selective oxidization layer, and a contact area between the semiconductor layer and an electrode metal of the current injecting unit can be enlarged. Therefore, thermal characteristic can be improved, series electrical resistance can be reduced, electric power consumption for driving can be lowered, and threshold current can be reduced.
The selective oxidization layer is typically composed of semiconductor including aluminum which is selectively or partially oxidized. The Al mole fraction of the selective oxidization layer can be changed with a desired distribution along a direction perpendicular to the substrate. The oxidization rate increases as the Al mole fraction increases. Thereby, a slope of the refractive index can be created in the selective oxidization layer, and the selective oxidization layer can act as a kind of lens. The light density in the active region is thus increased, and the threshold current is further reduced.
When the post portion is formed by removing the semiconductor material down to the uppermost level of the selective oxidization layer while the selective oxidization layer after selective oxidizing treatment is used as an etch stop layer, the oxidization width in an in-plane direction of the selective oxidization varies little per fabrication lot. Further, the selective ratio between the etch stop layer and the semiconductor material is increased, and therefore, etching can be stopped exactly at the uppermost level of the already-oxidized selective oxidization layer.
When the Al mole fraction of the selective oxidization layer is changed with a desired distribution along the direction perpendicular to the substrate, loss due to diffracted light in the current confinement layer can be reduced.
The growth substrate may be left untouched or may be entirely or partially removed. When the substrate is removed, oscillation light is not blocked by the substrate. The laser is therefore adaptable to a wide range of oscillation wavelengths. In this case, the current injection unit can be comprised of a metal formed on the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate, which are exposed due to formation of the post portion, and a metal formed on a bottom of the lower mirror layer exposed by the removal of the substrate. Accordingly, the heat radiation characteristic of the device can be further improved. Further, cross-talk between arrayed devices can be reduced.
The upper and lower mirror layers are typically composed of semiconductor multi-layer mirrors. When the substrate is removed, the upper mirror layer can be a semiconductor multi-layer mirror while the lower mirror layer can be a metal mirror formed on a face exposed by the removal of the substrate.
When the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate are covered with the electrode metal of the current injecting unit, current injection and heat dispersion can be effected through those sides as well. Therefore, the electrical resistance can be lowered, and the thermal conductivity can be enhanced in such a structure. The heat radiation characteristic can be further increased by thickening that electrode metal.
The current injection unit may be comprised of a metal formed on the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate, which are exposed due to formation of the post portion, and a metal formed on a bottom of the substrate. Alternatively, the current injection unit may be comprised of a metal formed on the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate, which are exposed due to formation of the post portion, and a metal formed on a bottom of the lower mirror layer (in the case where the substrate is removed). Still also the current injection unit may be comprised of a metal formed on the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate, which are exposed due to formation of the post portion, and a metal formed on the top of the substrate exposed by the partial removal of the layers on the substrate. Further, the current injection unit may be comprised of a metal formed on the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate, which are exposed due to formation of the post portion, and a metal formed on the lower mirror layer exposed by the partial removal of the layers on the substrate.
The active region can be comprised of quantum well semiconductor layers or the like.
When a plurality of surface emitting semiconductor lasers are arrayed on the substrate, the selective oxidization layer can act as an insulating layer for electrically separating the surface emitting semiconductor lasers from each other as well. The surface emitting semiconductor lasers can be arrayed with high density since the laser with the above structure has excellent heat radiation efficiency and thermal characteristics.
The surface emitting semiconductor laser can be bonded on a heat sink. Thereby, the thermal characteristics can be further improved. In this case, the surface emitting semiconductor laser is typically bonded on the heat sink with the upper mirror layer being on the side of the heat sink.
An electric wiring may be formed on the heat sink, and the electric wiring may be connected to a metal of the current injection unit formed on the upper mirror layer. In this case, a gold layer can be provided on the metal of the current injection unit formed on the upper mirror layer, and the electric wiring can be connected to the metal through the gold layer.
The present invention is also directed to a method of fabricating the above surface emitting semiconductor laser which includes the steps of growing semiconductor material on the substrate; partially removing the semiconductor material on the substrate down to an uppermost or halfway level of the selective oxidization layer while the selective oxidization layer prior to selective oxidizing treatment is used as an etch stop layer; and selectively oxidizing the selective oxidization layer to form a current confinement layer for the current injecting unit. In this fabrication method, an insulating layer for separating p-type and n-type layers from each other except a current can be formed as a result of formation of the current confinement layer. Therefore, no new insulating layer needs to be formed, and the fabrication process can be hence simplified.
Further, the present invention is also directed to a method of fabricating the above surface emitting semiconductor laser which includes the steps of growing semiconductor material on the substrate; forming a groove for partially oxidizing the selective oxidization layer; selectively oxidizing the selective oxidization layer to form a current confinement layer for the current injecting unit utilizing the groove; and partially removing the semiconductor material on the substrate down to an uppermost level of the selective oxidization layer while the selective oxidization layer after the selective oxidizing treatment is used as an etch stop layer. Also in this fabrication method, an insulating layer for separating p-type and n-type layers from each other except a current can be formed as a result of formation of the current confinement layer. Therefore, no new insulating layer needs to be formed, and the fabrication process can be hence simplified. In addition, the etch stop by the etch stop layer can be precisely achieved, and the fabrication yield can be hence improved. The groove is typically formed by etching the upper mirror layer, the selective oxidization layer prior to selective oxidizing treatment, and the active region down to the uppermost or halfway level of the lower mirror layer.
The selective oxidization layer is typically composed of AlxGa1xe2x88x92xAs (0.8xe2x89xa6xxe2x89xa61) formed in the vicinity of the active region. The fabrication method may further include a step of covering the top and sides of the upper mirror layer on the side of the active region opposite to the side of the substrate, which are exposed by the partial removing step, with a metal of the current injection unit.
These and other advantages will be more readily understood in connection with the following detailed description of the more preferred embodiments in conjunction with the drawings.